[G1 Android Development] ei8htohms:
Use some caution

It seems like a lot of folks are having some difficulty with this mod and perhaps a few have bricked their phones (if you can't get to recovery or bootloader even after pulling the battery AND SD card, your options are pretty limited), so please proceed with extreme caution.

Changing your SPL should not be taken lightly under any circumstances, because if it is corrupted or otherwise borked during the installation you may end up with a brick. The advice thrown around a lot previously about using a USB with a ferrite core is probably prudent (Google: "ferrite bead"), but more important and to the point is simply to learn how to use sha1sums and MD5 sums to verify that the package you downloaded AND the package that you copied to your SD card are exactly the same.

If the file corrupts during download or corrupts during the copy and you try to update with it, the likely next steps are asking T-Mo for a replacement phone. How to calculate and compare sha1sums and MD5 sums can be found with a little Googling, and there are a lot of free, easy to use apps for this purpose for Windows machines and presumably for Macs also (my understanding is that sha1sum is typically built into Linux distros, but I could be mistaken about that). Download these apps and learn how to calculate and compare these sums before changing your SPL. This point cannot be over-stressed IMHO.

Secondly, this SPL is radically different from all previous SPLs I've seen kicked around for the G1 because it recreates the partition sizes and therefor cannot just be updated and have the OS reboot and go on about its business (like all the previous SPLs). As far as I can tell, you have to run an update.zip of the ROM after running the update.zip of the SPL OR flash a Nandroid restore of the system and data partitions (minimum, probably boot too, but I'm not sure about that one) or else your OS will simply not boot (it will hang on the splash/boot screen).

If you're already conversant with fastboot to flash a Nandroid restore, then great; that's surely the easiest/quickest way to get the ROM back. If you're doing a lot of modding to your phone without knowing how to use Nandroid, then you're already playing with fire, but there is still a possibility for recovery in this situation. You'll have to use a card reader (or fastboot or ADB, but if you could do that you'd probably be flashing a Nandroid backup, right?) to get the ROM update.zip back onto our SD card so that you can reboot into recovery mode and re-apply it. This should not be a problem for most folks, but you'll feel a lot more comfortable digging around for a card reader before flashing the SPL than doing so while your phone (or worse yet, your girlfriend's phone) is hanging on the boot screen, you know?

Something else to note, while you can flash a previous Nandroid restore onto your phone after installing the new Haykuro modded SPL, should you change back to an older SPL (assuming they will recreate the partitions as originally intended), you will not be able to flash a Nandroid restore from a backup made while using the newer SPL. Since the system partition is bigger, it simply won't be able to fit onto the older partition size, you know? I have not tried this personally and don't really intend to for fear of joining the "Bricked G1" club, so feel free to post from experience if you have or simply educate me about what I might be missing if I'm mistaken.

Something else to note, if you are using custom boot images (splash pictures), the new SPL seems to eliminate splash2, at least when I tried to flash a splash2 image to my phone after updating the SPL, it gave me some kind of error about the partition not existing. It's possible I just messed something up, but I tried several times (and yes, I have performed this procedure multiple times with the older HardSPL) with no luck. This means you can only use a single boot screen now. :(

The new SPL is sweet and the new keyboard is "off the hook", but use some caution folks. We've all gotten a little cavalier about modding our phones, but unrecoverable calamities are a real possibility that should only be ignored at your own peril.